there are certainly differences.
the hd518, 558 and 598 are semi comparable but also different.
the 518 is the most basssy, not as neutral sound.
the 598 is the most neutral and flat sounding with the widest soundstage.
the 558 is midway between.
i did read an interesting teardown which seems to suggest that the drivers used in the hd518 are the same as the hd558 (or maybe the hd598 too not sure). make what you want of
http/www.head-fi.org/t/609634/hd-518-teardown-with-a-shocking-twist however modding them to sound better is more costly in this case than just getting the next step up.
for the price, the hd518 is a pretty decent sounding can although of course if you spend more money you can get something nicer sounding.
now, how do the hd518's compare to other sennheiser headphones? while i'm not familiar with the whole lineup i do know that they will be a whole lot more open sounding than the hd280 and other closed designs.
if you're looking for an alternative to the hd518... first i'd suggest having a larger budget and listing what you dont like and do like about the hd518 to help suggest something else. keep in mind most "better" cans than the hd518 are $110-200.